Beam cut-off circuit



J. F. BIGELOW BEAM CUT-OFF CIRCUIT Dec. 11, 1956 Filed June 27, 1952 V51? 7' PULSE INVENTOR. J E B/GELOW ATTORNEY,

BEAM CUT-OFF CIRCUIT John F. Bigelow, Fort Wayne, Iud., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application June 27, 1952, Serial No. 296,037

9 Claims. (Cl. 315-) This invention relates to an improved protective circuit particularly adapted for cutting off the beam of a cathode ray tube upon failure of deflection potential.

The development of so-called projection tubes, i. e. cathode ray tubes for television receiver use, having very high anode voltages and/or currents to permit a picture brightness of a magnitude to permit satisfactory operation with associated optical magnifiers, introduces the need for protective circuits to prevent damage to the phosphor screen upon failure of scanning potentials. Circuits heretofore known have utilized rectifiers to develop from the deflection pulses a relatively constant D. C. potential to so bias a control tube that the pcitrue tube beam is turned off in absence of such pulses. Such circuits have employed either electron tube rectifiers or crystal diodes. While some such rectifiers have performed satisfactorily, certain disadvantages reside therein such as initial and installation cost, spatial requirements, replacement costs and erratic operation of crystal rectifiers due to change in characteristics, all of which are substantially minimized by the present invention.

A broad object of the invention therefor resides in the provision of an improved protective circuit employing an inexpensive gas tube in a manner to serve as a rectifier to permit manufacture at a fraction of the cost heretofore encountered.

A further Object is to provide an improved circuit in which an inexpensive neon or similar bulb of long life expectancy may be employed as a rectifier. The bulb may be of a type which can be directly soldered to associated wiring, thus avoiding the expense of a socket and minimizing wiring.

A further object is to provide an improved protective circuit including a device functioning as a rectifier, and in which no heater power is required, as when a hot cathode vacuum tube rectified is employed.

Other advantages of the present invention will become obvious to persons skilled in the art upon examination of the drawings, the appended specification and the claims.

In the drawings, in which like parts are identified by the same reference numerals:

Fig. 1 illustrates an improved electron beam cut-off circuit with the principles of the present invention incorporated therein;

Fig. 2 is a fragmentary view of an alternate embodiment of the circuit of Fig. 1;

Fig. 3 illustrates another type of circuit in which the invention disclosed in Fig. 2 may be utilized.

Fig. 4 illustrates a second alternate embodiment of the inventive principles in the circuit for control of a cathode ray beam.

Referring to Fig. 1, the normal operation of the circuit shown, assuming uninterrupted sequential application of both horizontal deflecting pulses 39 and vertical deflecting pulses 32, and an applied video signal 10, is as follows:

The grid '21 of tube 19 receives negative bias from a source such as battery 22 sufiicient to establish cut-off atent in the absence of vertical pulses 32. Pulses 32 illustrated as negative going pulses, are fed through capacitorv 33, and rectified by a two element gas tube such as a neon bulb 24, or the like, connected through resistors 36, 35 to grid 21 and directly to ground. These pulses develop a positive voltage across resistor 27, connected between lead 25 and the negative terminal of battery 22, by reason of current flow through bulb 24 which closes a current loop comprising a resistor 27, battery 22, and bulb 24. This positive voltage is suflicient to lower the effective negative grid biasing voltage from battery 22 to render tube 19 conducting so long as vertical pulses 32 are sequentially applied.

Resistors 35 and 36, and capacitors 37 and 38 serve mainly as a ripple filter to smooth out the pulsating direct current voltage appearing across resistor 27. Since the plate resistance of tube 19 is low during conduction established by negative vertical pulses 32, the D. C. voltage at anode 18 is low and consequently the potential of cathode 12 of tube 13, derived from potentiometer 16, series connected with resistor 31 to a source of plate potential marked B+, and through dropping resistor 15, is also low. Tube 13 is the cathode ray tube to be protected, and is biased for possible conduction depending upon the applied video signal during said above condition.

The conditions above described assume continued application of horizontal deflecting pulses 39 through capacitor 4-9 to cathode 23 of tube 19 to leak off through resistor 25 to ground and thus drive cathode 23 negative. If desired, a positive cathode bias may be applied through resistor 41 as shown to reduce the negative bias from pulses 39, cathode 23 being maintained at the proper bias to ensure operation of the circuit as above described, with horizontal deflecting pulses 39 sequentially applied to cathode 23.

Upon failure of horizontal deflecting pulses 39, cathode 23 is not driven negative and the positive bias fed through resistor 41 appears at the cathode end of resistor 25, with tube 19 rendered close to, or below the point of plate current cut-off, with the plate resistance increasing to a high value. With increased plate resistance of tube 19, the D. C. potential across potentiometer 16 rises appreciably, with a consequent rise of potential at cathode 12 suflicient to render tube 13 cut-off to prevent the electron beam from striking the picture tube phosphor.

It is contemplated that in some instances the operating and striking potentials of a particular gas discharge tube employed may be either too high or too low in respect to the magnitude of the deflection pulses to be rectified therein to permit use of the tube without biasing means directly associated therewith. As shown in Fig. 2, assuming that the magnitude of the negative going vertical pulses 32 is less than the striking potential of bulb 24, a biasing battery 45 may be series connected as shown between the lower tube element and ground to adjust the striking potential to the proper level. Under other circumstances, it may be desirable to employ a battery, reversed in potential, to raise the striking voltage of the bulb 24, and it is within the scope of the invention to employ plural bulbs 24 series connected to ground to alter the striking and operating potentials between the grid input circuit and ground under certain conditions. The operation of the glow discharge tube as a rectifier under these conditions will be readily understood in view of the above explanation. While deflection pulses usually are of a single polarity, such as purely negative pulses, slightly positive excursions may occur, and while bu-lb 24 normally does not serve as a half-wave rectifier, however, should the deflection pulses contain slightly positive excursions, they would serve merely to lower the potential gradient between the electrodes of bulb .24, the grounded the .cathode positive as shown. A glow type gas tube 124- and a biasing battery '56 are series connected between lead 48 and ground for rectification of pulses '39 in the manner taught to develop a positive grid potential across resistor 53. Tube '52 is maintained conducting in the presence of applied pulses 39 which develop a sufficient positive potential across resistor 53 to maintain tube 2 above cut-ofi with high plate current flowing through relay 55 to close relay contact 56, conductors 57 being associated with the beam circuit to maintain the beam on with contact 56 closed. Upon failure of sequentially applied pulses 39, the positive potential across resistor 53 is reduced to zero, rendering grid 5-1 at a high negative potential in respect to the cathode, hence cut-otf swings to a negative value rendering tube 52 cut-off to terminate plate current in tube 55, thus allowing contact '56 to open and break the circuit associated with conductor 57.

Fig. 4 illustrates a modification of the invention with positive input pulses 44 fed through transformer 60 and over lead 61 to an associated yoke. Rectification of positive pulses 44 is effected by glow discharge tube 63, the pulses exceeding the striking potential of tube 63, to develop a positive potential across resistor 64 series connected between the lower element of tube 63 and ground. The development of potential across resistor 64, of a high value, charges condenser 65, and upon cessation of pulses 39 the condenser 65 discharges slowly through resistor 64 to gradually reduce the charge thereon. It is thus seen that during sequential application of pulses 44, a fairly uniform positive potential is maintained at lead 66 which may be employed in the manner taught above for control of a protective circuit, for example, by direct control of a gating tube as described in respect to the circuit of Fig. 3. Upon failure of pulses 39, capacitor 65 discharges through resistor 64 to terminate the normal positive voltage at lead 60. Capacitor 65 does not normally discharge through bulb 63 between pulses, since the time constant of resistor 64 and capacitor 65 is so chosen that the potential across resistor 64 does not approach the striking potential of bulb 63, the charge time being short with respect to the discharge time.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. In a protective circuit for a cathode ray tube having a beam source, beam deflection means, and a deflection pulse source whereby the absence of said pulses results in extinguishment of the beam, an electron discharge device operative between a state of conduction and a state of non-conduction connected for the control of said beam in response to the presence or absence of deflection pulses, and means to initiate and terminate conduction through said discharge device in response to the presence or absence of applied deflection pulses including a two element.

gas tube series connected between the grid and cathode of said control tube, a resistor shunting said gas tube,

4. diode rectifier, and means connecting said deflection pulse source to that element of said gas tube connected to said grid.

2. In a protective circuit in which a sequence of deflection pulses are employed to sweep an electron beam, the absence of said pulses resulting in extinguishment of the beam, an electron discharge device operative between a state of conduction and a state of non-conduction for the control of said beam in response to the presence or absence of deflection pulses, and means to initiate and terminate conduction through said discharge device in response to the presence or absence of applied deflection pulses including a two element gas tube series connected between the grid and cathode of said control tube, a resistor shunting said gas tube, and means for applying deflection pulses to that element of said gas tube connected to said grid.

3. The device of claim 2 including bias means series connected between said resistor and said cathode.

4. In a device of the character described, a cathode ray tube, a source of deflection pulses, an electron discharge device including a cathode, an anode and a grid,

means for the introduction of deflection pulses to said grid, a two electrode gas tube connected between said grid and cathode, and a resistor connected between said cathode and grid.

5. In a protective circuit for a cathode ray tube having.

a beam control grid, and beam deflection means, the combination with an electron discharge device including anode, cathode and grid, the anode being connected to control the bias of said beam control grid and means for sequentially applying deflection pulses to the grid of said device, series connected between said cathode and ground to maintain said cathode at a potential above ground, a resistor connected between said grid andground, and a two electrode gas tube having one element connected to said grid, and means for supplying a biasing potential connected between the second element of said gas tube and ground.

6. In a protective circuit, a cathode ray tube, a source of deflection pulses an electron discharge device including a cathode, an anode, and grid, means connecting said cathode to ground, means for introducing a sequence of deflection pulses to said grid, a two electrode gas tube having one electrode connected to said grid, biasing means series connected between the second element of said gas tube and ground, a resistor having one end connected to said grid, and a second source of biasing potential series connected between the opposite end of said resistor and ground, said anode being connected to the beam control grid-electrode of said cathode ray tube to cut otf said beam when beam deflections fail.

7. In a television receiving system of the type in which the anode of a cathode ray tube is energized by a source of high positive potential to maintain the beam, and means are provided to interrupt said source to turn the beam ofl upon failure of sequentially applied deflection pulses, means including an electron discharge device for connecting, when in a conductive state, said source to said anode, and disconnecting said source from said anode when in a non-conductive state, a second electron dis means for the control of conduction through said second device in response to the presence or absence of a sequence of deflection pulses, said last mentioned means including a two element gas tube, means connecting one element of said tube to the control element of said second device, a resistor connected to shunt the electrodes of said gas tube, and means connecting the second elec-= trode of said tube and said resistor to the cathode of said second device, whereby sequentially applied deflecting pulses of predetermined amplitude are rectified by said gas tube to establish a potential across said shunted resistor to bias the second device, hence the first device, conducting, the control element of said second device being normally biased in the absence of a pulse-initiated voltage across said resistor, to cut-01f to thereby establish cut-oif of said first device.

8. A protective device for a television system which operates to extinguish the picture tube beam during the period of time no beam-deflecting signals are applied comprising an electron discharge device having control, anode and cathode electrodes, first means for biasing said control electrode to a potential below cut-off to prevent said electron discharge device from conducting a space current, rectifier means operative in response to beamdeflecting impulses to provide a unipotential signal, means operatively coupling said rectifier means to said control electrode, the unipotential signal of said rectifier means being of such polarity and magnitude as to cause said electron discharge device to conduct only during the time said impulses are applied to said rectifier means, second means operatively coupled to said cathode electrode for applying a cut-oil bias to said control electrode, and means operatively coupled to said cathode electrode for applying other beam-deflecting impulses thereto, these latter impulses serving to reduce the bias provided by said second means and thereby cause conduction of said electron discharge device.

9. A protective device for a television system which operates to extinguish the picture tube beam during the period of time no beam-deflecting signals are applied comprising a unidirectional current-conducting device having a current-controlling element, first means for biasing said element to a potential which places said device in one state of conduction, rectifier means operative in response to first beam-deflecting impulses to provide a unipotential signal, and means operatively coupling said rectifier means to said current-controlling element, said unipotential signal being of such polarity and magnitude as to place said device in said one state of conduction, and second means for biasing said element between a potential which sustains said one state of conduction and a second potential which places said device in an opposite state of conduction, said second means including circuitry which operatively couples second beam-deflecting impulses to said current-controlling element, said second impulses being of a polarity and magnitude to sustain said one state of conduction.

References Cited in the file of this patent UNITED STATES PATENTS 2,097,272 Drake Oct. 26, 1937 2,119,372 Wendt May 31, 1938 2,152,821 Schlesinger Apr. 4, 1939 2,153,216 Urtel Apr. 4, 1939 2,158,261 Urtel et al. May 16, 1939 2,210,702 Bownam-Manifold Aug. 6, 1940 2,280,670 Spielman Apr. 21, 1942 2,369,631 Zanarini Feb. 13, 1945 2,506,723 Larsen May 9, 1950 2,532,566 Millman Dec. 5, 1950 2,536,712 Bentley Ian. 2, 1951 2,584,932 Snyder et al. Feb, 5, 1952 

